This invention relates in general to electric motors and in particular to a variable reluctance motor having coils which are wound from foil wire.
Electric motors are well known devices which convert electrical energy to rotary mechanical energy. To accomplish this, electric motors establish and control electromagnetic fields so as to cause the desired rotary mechanical motion. There are many different types of electric motors, each utilizing different means for establishing and controlling the electromagnetic fields. Consequently, the operating characteristics of these electric motors vary from type to type, and certain types of electric motors are better suited for performing certain tasks than others.
Synchronous motors constitute one principal class of electric motors. The two basic components of a synchronous motor are (1) a stationary magnetic field generating structure, generally referred to as the stator, and (2) a rotatable component driven thereby, generally referred to as the rotor. Both the stator and the rotor are usually formed from magnetic materials, such as iron. Synchronous motors are characterized in that the rotational speed of the rotor is directly related to the frequency of the electrical input signal applied thereto. Thus, so long as the frequency of the applied electrical input signal is constant, the rotor will be driven at a constant rotational speed. Within this broad definition, however, the structure and operation of synchronous electric motors vary widely.
One variety of synchronous electric motor is known as a variable reluctance (VR) motor. VR motors operate on the principle that a magnetic field surrounding a magnetic material component will exert a mechanical force on that component, urging it to become aligned with the magnetic flux lines generated by the magnetic field. Thus, by using the stator to establish and rotate a magnetic field about the rotor, the rotor can be rotatably driven to move relative to the stator. In a basic VR motor structure, this can be accomplished by providing one pair of opposed magnetic poles on the stator and a corresponding pair of opposed magnetic poles on the rotor. A coil of electrically conductive wire is wound about each of the two stator poles. By passing electrical current through each of the stator coils in an appropriate manner, the stator poles can be selectively electro-magnetized so as to attract the corresponding poles of the rotor thereto.
Frequently, two or more pairs of poles are provided on both the stator and the rotor. In this more advanced VR motor structure, electrical current is passed in sequential fashion through the stator coils so as to attract the corresponding rotor poles thereto. By providing more poles on the stator and the rotor, the overall torque generated by the VR motor can be increased. Also, the additional poles facilitate rotation of the rotor at a more uniform speed.
To further optimize the operation of the VR motor, the magnitude of the electrical current which is sequentially passed through the stator coils can be varied as a function of the rotational displacement of the rotor, as opposed to simply being supplied in an on-off manner. For example, the magnitude of the electrical current passed through a particular stator coil can initially be large, but decrease as the rotor pole rotates toward it. Consequently, the stator coil is prevented from continuing to attract that rotor pole toward it after the rotor pole has rotated past the stator pole.
As mentioned above, the electromagnetic coils used in VR motors are formed by winding electrically conductive wire about the poles of the stator. The wire used in such windings has typically been conventional wire having a circular cross sectional shape. While such wire is commonly available and relative inexpensive, the circular cross sectional shape of the wire does not lend itself to high density winding. In other words, the circular cross sectional shape of the wire causes a relatively large number of gaps to be created between adjacent turns of the wire when the coil is wound. As a result, the overall winding density (i.e., the number of windings per unit space) of the coil is limited. Since the strength of the electromagnetic field generated by the stator (for an acceptable power dissipation of the winding) is directly related to this winding density, the strength of the electromagnetic field is also limited (because of thermal considerations). Accordingly, it would be desirable to provide an improved electromagnetic coil structure for a VR motor which maximizes the overall winding density of the coil windings so as to maximize the strength of the electromagnetic field generated thereby.